Because there are so many causes of cerebellar damage (e.g. alcoholism, blast injury, neurodegenerative diseases, stroke, and simple aging), veterans suffering imbalance, visual impairment, and incoordination due to cerebellar damage are common. In the past, neurologists had few therapies to improve function in these patients. Now two emerging ideas in cerebellar physiology hold the promise that better treatments can be rationally designed. The irregularity hypothesis states that cerebellar dysfunction arises when cerebellar Purkinje cells (PCs) fire in irregular patterns through loss of their pacemaker properties. It is cited to explain why drugs that increase PC rhythmicity in vitro such as 4-aminopyridine (4-AP) improve certain manifestations of cerebellar disease in mice and humans, and it predicts their usefulness in a wide range of cerebellar disorders. The PC synchrony hypothesis states that synchrony of firing across multiple PCs determines the effectiveness with which PCs control their synaptic targets, and may explain why PC irregularity ? which could disrupt PC synchrony ? is deleterious. If correct, these hypotheses indicate how laboratory assays can be used to develop more tolerable and effective drugs. If incorrect, their application to drug development will be futile. Currently, both hypotheses are unproven, and there are data challenging the applicability of the theory to the flocculus and other regions of the vestibulocerebellum, even though it was work in the flocculus that led to the irregularity hypothesis in the first place. This project will address conflicting findings in the literatures on irregularity, 4-AP, and PC synchrony. Like much previous work on the irregularity hypothesis, parts of the proposal will be conducted in the ataxic mouse tottering (tg), which carries a mutation in Cacna1a, the gene encoding the ion pore subunit of the P/Q calcium channel. We focus on the flocculus and its control of reflex eye movements that maintain clear vision, because their physiology is well understood, because work in this area provides both support and challenges to the irregularity and synchrony hypotheses, because eye movement and related balance abnormalities contribute significantly to the symptoms of cerebellar disease, and because successes to date predict their treatment is possible. Specific Aim 1 will investigate why bath-applied 4-AP restores regularity of tg vermis PCs in vitro, and oral 4-AP improves tg's performance on the rotarod, but parenterally administered 4-AP does not improve tg's eye movement deficits that are attributed to flocculus dysfunction. We will test the possibilities that the conundrum arises through non-validity of the irregularity hypothesis, or through regional variations in cerebellar physiology, or through differing effects of chronic oral vs. short-term parenteral exposure to 4-AP. Specific Aim 2 addresses the idea that PC irregularity disrupts the PC-PC synchrony on which normal cerebellar function depends. We will test that explanation by determining whether PC synchrony is in fact reduced in tg. As in Aim 1, to address the possibility of regional variations of physiology, we will record in the flocculus and in a non- vestibulocerebellar region of the vermis. Specific Aim 3 addresses a prediction of the posited linkage between irregularity and PC synchrony: If these ideas apply to the flocculus, then variations in PC firing rate should drive eye movements more robustly when the PCs fire more synchronously. Making use of an optogenetic mouse strain whose PCs express channelrhodopsin, we will stimulate PCs with patterns of photostimulation predicted to trigger PC firing with varying degrees of synchrony. Through recordings of PC firing rates and eye movements, we will quantify and compare the efficiency with which flocculus signals are transferred to subsequent circuitry. The results of this proposal will have broad implications for the general validity of the irregularity hypothesis, the usefulness and limitations of in vitro regularity assays in drug development, and predicting which cerebellar disorders might respond to 4-AP.